By way of background, a short introduction to diamond materials science is presented here in order to set the context for the present invention.
Diamond materials are based on a theoretically perfect diamond lattice. The properties that would be exhibited by this theoretically perfect lattice are well understood. For example, such a theoretically perfect diamond lattice would exhibit extremely high thermal conductivity, low electrical conductivity (very wide band gap intrinsic semi-conductor with no significant charge carriers but with high charge carrier mobility if charge carriers are introduced into the lattice structure), extremely low thermal expansion coefficient, no significant optical birefringence, and low optical absorption (no significant absorption in the visible spectrum so there would be no colour).
Such a theoretically perfect diamond lattice is thermodynamically impossible to attain. In reality, it is practically difficult to even approach a level of perfection which would be possible to achieve in theory when taking into account thermodynamic considerations. As such, it should be apparent that all diamond materials contain a significant number of defects. Such defects may come in the form of impurities. Typical impurities which may be incorporated into a diamond lattice structure include nitrogen, boron, silicon, phosphorous, hydrogen, and metals such as sodium, nickel, and chromium. Additionally, defects within diamond materials also include crystallographic deviations from the perfect diamond lattice structure in the form of point defects such as vacancies and interstitials and extended defects such as various forms of dislocation defects. Defects may also combine in various ways. For example, vacancy defects may combine into clusters or combine with impurity atoms to form unique vacancy structures with their own individual properties. Examples include silicon containing defects such as silicon-vacancy defects (Si—V), silicon di-vacancy defects (Si—V2), silicon-vacancy-hydrogen defects (Si—V:H), silicon di-vacancy hydrogen defects (S—V2:H) and nitrogen containing defects such as nitrogen-vacancy defects (N—V), di-nitrogen vacancy defects (N—V—N), and nitrogen-vacancy-hydrogen defects (N—V—H). These defects are typically found in a neutral charge state or in a charged state, e.g. negatively charged.
Defects within diamond materials significantly alter the properties of the materials. Ongoing work in this field is concerned with understanding the properties of the various defects within diamond materials and their overall effect on the functional properties of the materials. Furthermore, ongoing work is concerned with engineering diamond materials to have particular types and distributions of defects in order to tailor diamond materials to have particular desirable properties for particular applications. The types and distributions of defects which are desired will thus depend on the properties required for particular applications.
In this regard, diamond materials may be categorized into three main types: natural diamond materials; HPHT (high pressure high temperature) synthetic diamond materials, and CVD (chemical vapour deposited) synthetic diamond materials. These categories reflect the way in which the diamond materials are formed. Furthermore, these categories reflect the structural and functional characteristics of the materials. This is because while natural, HPHT synthetic, and CVD synthetic diamond materials are all based on a theoretically perfect diamond lattice the defects in these material are not the same. For example, CVD synthetic diamond contains many defects unique to the process of CVD, and whilst some defects are found in other diamond forms, their relative concentration and contribution is very different. As such, CVD synthetic diamond materials are different to both natural and HPHT synthetic diamond materials.
Diamond materials may also be categorized according to their physical form. In this regard, diamond materials may be categorized into three main types: single crystal diamond materials; polycrystalline diamond materials; and composite diamond materials. Single crystal diamond materials are in the form of individual single crystals of various sizes ranging from small “grit” particles used in abrasive applications through to large single crystals suitable for use in a variety of technical applications as well for gemstones in jewelry applications. Polycrystalline diamond materials are in the form a plurality of small diamond crystals bonded together by diamond-to-diamond bonding to form a polycrystalline body of diamond material such as a polycrystalline diamond wafer. Such polycrystalline diamond materials can be useful in various applications including thermal management substrates, optical windows, and mechanical applications. Composite diamond materials are generally in the form of a plurality of small diamond crystals bonded together by a non-diamond matrix to form a body of composite material. Various diamond composites are known including diamond containing metal matrix composites, particularly cobalt metal matrix composites known as PCD, and skeleton cemented diamond (ScD) which is a composite comprising silicon, silicon carbide, and diamond particles.
It should also be appreciated that within each of the aforementioned categories there is much scope for engineering diamond materials to have particular concentrations and distributions of defects in order to tailor diamond materials to have particular desirable properties for particular applications. In this regard, the present invention is concerned with CVD synthetic diamond materials to which the focus of this specification will now turn.
CVD processes for synthesis of diamond material are now well known in the art. Useful background information relating to the chemical vapour deposition of diamond materials may be found in a special issue of the Journal of Physics: Condensed Matter, Vol. 21, No. 36 (2009) which is dedicated to diamond related technology. For example, the review article by R. S Balmer et al. gives a comprehensive overview of CVD diamond materials, technology and applications (see “Chemical vapour deposition synthetic diamond: materials, technology and applications” J. Phys.: Condensed Matter, Vol. 21, No. 36 (2009) 364221).
Being in the region where diamond is metastable compared to graphite, synthesis of diamond under CVD conditions is driven by surface kinetics and not bulk thermodynamics. Diamond synthesis by CVD is normally performed using a small fraction of carbon (typically <5%), typically in the form of methane although other carbon containing gases may be utilized, in an excess of molecular hydrogen. If molecular hydrogen is heated to temperatures in excess of 2000 K, there is a significant dissociation to atomic hydrogen. In the presence of a suitable substrate material, CVD synthetic diamond material can be deposited. Polycrystalline CVD diamond material may be formed on a non-diamond substrate such as a refractory metal or silicon substrate. Single crystal CVD synthetic diamond material may be formed by homoepitaxial growth on a single crystal diamond substrate.
Atomic hydrogen is essential to the process because it selectively etches off non-diamond carbon from the substrate such that diamond growth can occur. Various methods are available for heating carbon containing gas species and molecular hydrogen in order to generate the reactive carbon containing radicals and atomic hydrogen required for CVD synthetic diamond growth including arc-jet, hot filament, DC arc, oxy-acetylene flame, and microwave plasma.
Impurities in the CVD process gases are incorporated into the CVD synthetic diamond material during growth. As such, various impurities may be intentionally introduced into the CVD process gases, or intentionally excluded from the CVD process gases, in order to engineer a CVD synthetic diamond material for a particular application. Furthermore, the nature of the substrate material and the growth conditions can affect the type and distribution of defects incorporated into the CVD synthetic diamond material during growth.
For certain applications it is desirable to minimize the number of defects, or at least certain types of defect, within the diamond lattice structure. For example, for certain electronic applications such as radiation detectors or semi-conductive switching devices it is desirable to minimize the number of charge carriers inherent in the diamond material and increase the mobility of charge carriers intentionally introduced into the material in use. Such a material may be engineered by fabricating a single crystal CVD synthetic diamond material which has a low concentration of impurities which would otherwise introduce charge carriers into the diamond lattice structure. Patent literature relevant to such electronic/detector grade single crystal CVD synthetic diamond material includes WO01/096633 and WO01/096634.
For certain optical applications it is desirable to provide a material which has low optical absorbance and low optical birefringence. Such a material may be engineered by fabricating a single crystal CVD synthetic diamond material which has a low concentration of impurities, which would otherwise increase the optical absorbance of the material, and a low concentration of extended defects which would otherwise introduce anisotropic strain into the diamond lattice structure causing birefringence. Patent literature relevant to such optical grade single crystal CVD synthetic diamond material includes WO2004/046427 and WO2007/066215.
High purity diamond material is also desirable to function as a host material for quantum spin defects in certain quantum sensing (e.g. in measuring magnetic fields) and processing applications Diamond materials are useful in such applications as certain quantum spin defects (e.g. the negatively charge nitrogen-vacancy defect) disposed within the diamond lattice structure have a long decoherence time even at room temperature (i.e. the quantum spin defects remain in a specific quantum spin state for a significant length of time allowing sensing and/or quantum processing applications to be performed). Furthermore, such quantum spin defects within the diamond lattice can be optically addressed. However, in such applications impurities can interact with quantum spin defects within the diamond lattice structure reducing their decoherence time and thus reducing their sensitivity and/or reducing the time during which quantum processing applications can be performed. Patent literature relevant to such high purity quantum grade single crystal CVD synthetic diamond material includes WO 2010010344 and WO 2010010352.
In contrast to the low defect materials described above, for certain applications it is desirable to intentionally introduce a significant but controlled quantity, type and distribution of defects into the diamond lattice structure. For example, introducing boron into the diamond lattice by providing a boron containing gas within the CVD process gases provides an acceptor level within the band structure of the diamond material thus forming a p-type semi-conductor. If extremely high levels of boron are introduced into the diamond lattice structure the material shows full metallic conductivity. Such materials are useful as electrodes, as electrochemical sensing electrodes, and in electronic applications. Patent literature relevant to such boron doped single crystal CVD synthetic diamond material includes WO03/052174.
Another example is the addition of nitrogen to high-pressure high-temperature (HPHT) synthetic diamond materials. It is well-known that high concentrations (hundreds of parts per million) of atomic nitrogen can be incorporated into HPHT synthetic diamond. However for several applications, HPHT-grown diamond possesses additional qualities that are detrimental. Growth tends to be highly non-uniform with a higher defect impurity (nitrogen as well as trace metals) in some sectors compared to others, and hence HPHT-grown diamond commonly exhibits colour zoning in both its as-grown and treated states. Non-uniformity along with stacking faults along the sector boundaries can also influence the fracture toughness of the material produced. Also, commonly present in HPHT-grown diamond material are metal inclusions, as a consequence of the solvent metal used as a catalyst in the HPHT growth process. These metal inclusions can strongly affect the mechanical properties of the material produced.
Another example, particularly pertinent to the present invention, is that of nitrogen doped single crystal CVD synthetic diamond materials. Nitrogen is one of the most important dopants in CVD diamond material synthesis as it has been found that providing nitrogen in the CVD process gas increases the growth rate of the material and can also affect the formation of crystallographic defects such as dislocations. As such, nitrogen doping of single crystal CVD synthetic diamond materials has been extensively investigated and reported in the literature. Nitrogen doped CVD synthetic diamond material tends to be brown in colour. As such, for the previously discussed applications, such as optical applications, it has been found to be advantageous to develop techniques which intentionally exclude nitrogen from the CVD process gases. However, for applications such as mechanical applications where optical, electronic, and quantum coupling parameters are not a concern, nitrogen doping to significant levels can be useful in achieving growth of thick layers of CVD synthetic diamond material. Patent literature relevant to such nitrogen doped single crystal CVD synthetic diamond material includes WO2003/052177 which describes a method of fabricating diamond material using a CVD synthesis atmosphere comprising nitrogen in a concentration range 0.5 to 500 ppm, calculated as molecular nitrogen.
Nitrogen doped single crystal CVD synthetic diamond material has also been found to be a useful starting material for post-growth treatments such as irradiation and/or annealing to achieve desirable colours. For example, WO2004/022821 describes an annealing technique which may be applied to yellow/brown nitrogen doped single crystal CVD synthetic diamond material to achieve desirable colours such as pinks, greens, colourless and near colourless. Such treated single crystal CVD synthetic diamond material may have jewelry applications as gem stones. Further irradiation and annealing techniques for converting nitrogen containing single crystal CVD synthetic diamond material into desirable colours are described in WO 2010149777 (to produce orange single crystal CVD synthetic diamond material), WO 2010149775 (to produce light pink single crystal CVD synthetic diamond material), and WO 2010149779 (to produce light blue single crystal CVD synthetic diamond material). These treatments involve single crystal CVD synthetic diamond materials having various levels of single substitutional nitrogen, single substitutional vacancy defects (neutral and negatively charged), and nitrogen-vacancy defects. The defect centres that cause colour commonly luminesce as well, and therefore the post-growth treatment of diamond in this way allows the engineering of luminescent centres which may be used for e.g. diamond-based dyes.
In addition to the above, US2011/0151226 describes that there is a need for a single crystal CVD synthetic diamond material with a relatively high nitrogen content that is uniformly distributed and which is free of other defects, such as inclusions, normally associated with HPHT synthesis processes. In this regard, US2011/0151226 describes a CVD growth process which uses a CVD process gas including nitrogen and oxygen containing gases in addition to the standard carbon and hydrogen containing gases. These process gases are included at certain specified ratios to obtain CVD synthetic diamond material with both a high concentration of nitrogen in the form of single substitutional nitrogen and a low concentration of other defects. It is described that such a growth chemistry is advantageous for producing material having a colour which is not dominated by brown defects but is instead dominated by a yellow hue due to the presence of single substitutional nitrogen. It is further described that the electronic properties of the material are dominated by single substitutional nitrogen, but not degraded by the other defects normally resulting from nitrogen in the growth process and that the material may be used for gem applications and for technical applications such as in electronics and radiation detectors.
US2011/0151226 uses a CVD synthesis atmosphere containing nitrogen at an atomic concentration in a range 0.4 ppm to 50 ppm. Furthermore, it is described that for the duration of the synthesis process the substrate on which the single crystal CVD synthetic diamond material is grown is maintained at a temperature in the range 750° C. to 1000° C. It is described that this process is capable of synthesizing CVD diamond material comprising single substitutional nitrogen (Ns0) at a concentration of greater than about 0.5 ppm and having a total integrated absorption in the visible range from 350 nm to 750 nm such that at least about 35% of the absorption is attributable to Ns0.
Zhang et al., Diamond & Related Materials, 20, 496-500 (2011) also disclose a CVD growth process using a process gas which includes nitrogen and oxygen containing gases in addition to hydrogen and carbon containing species. The described process utilizes a substrate temperature of 1000° C. It is taught that the addition of CO2 can actually reduce the concentration of nitrogen incorporation into the CVD synthetic diamond material.
In addition to the above, a number of additional prior art documents discuss various CVD diamond synthesis processes which utilize one or more of nitrogen process gas, high substrate temperature, and oxygen process gas. These are briefly discussed below.
U.S. Pat. No. 7,883,684 discloses a CVD diamond synthesis method which uses a synthesis atmosphere comprising 8% to 20% CH4 per unit of H2 and 5% to 25% O2 per unit of CH4. It is also described that the gas mix can optionally include 0.2% to 3% N2 per unit of CH4. It is stated that the addition of N2 to the gas mix at this concentration creates more available growth sites, enhances the growth rate, and promotes {100} face growth. It is further described that the method includes controlling the temperature of a growth surface of the growing single crystal CVD synthetic diamond material at a growth temperature in the range 700° C. to 1100° C. For the examples which utilize nitrogen it is stated that the resultant single crystal CVD synthetic diamond material is brown in colour and that the colour of the material can be changed by annealing.
U.S. Pat. No. 7,820,131 discloses a CVD diamond synthesis method which uses a synthesis atmosphere comprising 8% to in excess of 30% CH4 per unit of H2 and optionally 5% to 25% O2 per unit of CH4 to produce a colourless single crystal CVD synthetic diamond material. It is also described that a gas mix which comprises nitrogen rather than oxygen results in a single crystal CVD synthetic diamond material which is brown in colour. It is further described that the method includes controlling the temperature of a growth surface of the growing single crystal CVD synthetic diamond material at a growth temperature in the range 900° C. to 1400° C.
US2010/0126406 also discloses a CVD diamond synthesis method which uses a synthesis atmosphere comprising hydrogen, a carbon source, and an oxygen source. Two alternative embodiments are described: (i) a process in which the synthesis atmosphere is essentially free of nitrogen resulting in the growth of colourless single crystal CVD synthetic diamond material; and (ii) a process in which the synthesis atmosphere includes a small amount of nitrogen resulting in the growth of brown single crystal CVD synthetic diamond material.
U.S. Pat. No. 7,157,067 discloses a CVD diamond synthesis method which uses a synthesis atmosphere comprising hydrogen, a carbon source, and nitrogen with a N2/CH4 ratio of 0.2% to 5.0% and a CH4/H2 ratio of 12% to 20%. It is described that by using such a synthesis atmosphere and controlling the temperature of a growth surface of the growing single crystal CVD synthetic diamond material at a growth temperature in the range 1000° C. to 1100° C. it is possible to produce single crystal CVD synthetic diamond material with increased fracture toughness.
US2009/0038934 discloses a CVD diamond synthesis method which uses a synthesis atmosphere which includes oxygen. It is further described that optionally the synthesis atmosphere comprises hydrogen, methane at a concentration of 6% to 12% per unit of hydrogen, nitrogen at a concentration of 1% to 5% per unit of hydrogen, and oxygen at a concentration of 1% to 3% per unit of hydrogen. It is further described that the temperature of a growth surface of the growing single crystal CVD synthetic diamond material is controlled at a growth temperature in the range 900° C. to 1400° C.
JP2008110891 discloses a CVD diamond synthesis method which uses a synthesis atmosphere comprising atomic concentrations of carbon to hydrogen of 2% to 10%, nitrogen to carbon of 0.1% to 6%, and oxygen to carbon of 0.1% to 5%.
JP7277890 discloses a CVD diamond synthesis method which uses a synthesis atmosphere comprising hydrogen, carbon, nitrogen and optionally oxygen or boron. It is further disclosed that diamond having 3-1,000 ppm ratio of the number of nitrogen atoms to that of hydrogen atoms or 3-100% ratio of the number of oxygen atoms to that of carbon atoms is synthesized. It is described that since a very small amount of nitrogen is added as gaseous starting material, high quality diamond is synthesized at an increased rate of synthesis.
U.S. Pat. No. 6,162,412 discloses a CVD diamond synthesis method which uses a synthesis atmosphere in which a concentration of carbon atoms in relation to hydrogen gas (A %), a concentration of nitrogen gas in relation to the whole reaction gas (B ppm) and a concentration of oxygen atoms in relation to the hydrogen gas (C %) satisfies the equation: α=(B)1/2×(A−1.2C), provided that α is not larger than 13 or B is not larger than 20. The examples indicate that the substrate was held at a temperature of 950° C. during CVD diamond growth. It is further stated that the synthesized CVD diamond material contains 20 ppm or less of nitrogen.
Chayahara et al. “The effect of nitrogen addition during high-rate homoepitaxial growth of diamond by microwave plasma CVD” Diamond & Related Materials 13, 1954-1958 (2004) discloses a CVD diamond synthesis method which uses a synthesis atmosphere comprising 500 sccm hydrogen, 40 sccm methane, and nitrogen from 0 to 3 sccm. Two different substrate temps are disclosed—1220° C. for an open type substrate holder and 1155° C. for an enclosed type holder. It is described that nitrogen increases growth rate and alters the surface morphology of the CVD synthetic diamond material.
Mokuno et al. “High rate homoepitaxial growth of diamond by microwave plasma CVD with nitrogen addition” Diamond & related Materials 15, 455 to 459 (2006) discloses a CVD diamond synthesis method which uses a synthesis atmosphere comprising 500 sccm hydrogen, 60 sccm methane, and nitrogen from 0.6 to 1.8 sccm. As in the previously discussed paper two different substrate holders were used, one being an open type holder and one being a closed type holder. Substrate temperatures in a range 1060° C. to 1250° C. are disclosed. It is reported that nitrogen concentrations in the CVD synthetic diamond materials formed using these process parameters range from 8.9 to 39 ppm.
Chayahara et al. “Development of single-crystalline diamond wafers” Synthesiology, vol. 3, no. 4, 259-267 (2011) discloses a similar CVD diamond synthesis method which uses a synthesis atmosphere comprising 500 sccm hydrogen, 60 sccm methane, and nitrogen from 0 to 3 sccm with substrate temperatures in a range 1100° C. to 1200° C.
In light of the above, it is evident that the prior art relating to nitrogen doping in CVD diamond synthesis process is reasonably extensive. In the context of this prior art, the present inventors have investigated routes to achieve high levels of nitrogen incorporation into CVD synthetic diamond materials. As such, the present inventors have been particularly interested in ‘high’ nitrogen gas fraction/‘high’ substrate temperature CVD diamond synthesis processes, ‘high’ being defined as substantially greater than ‘standard’ diamond growth that takes place at 700-950° C. with nitrogen gas fractions of, for example, less than 20 ppm of the gas mix. The present inventors have found that high nitrogen gas fraction/high substrate temperature growth conditions allow substantially greater concentrations of single substitutional nitrogen defects (Ns) to be incorporated into the CVD synthetic diamond material (e.g. 5 to 50 ppm) than standard growth conditions, along with a significant concentration of as-grown nitrogen-vacancy defects (e.g. approximately 100 ppb). Such material is useful for a range of applications including certain quantum sensing and processing applications, optical filters, mechanical tool pieces, and as a starting material for post-growth irradiation and/or annealing treatments to form coloured gemstones. In relation to quantum sensing and processing applications, it was previously described that high purity diamond material is desirable for such applications in order to achieve long decoherence times. However, for certain quantum sensing applications, such as magnetometry, sensitivity is related to the product of the density of NV− defects and the decoherence times of these defects. In such circumstances, it can be desirable to provide a large concentration of NV− centres for certain applications even if the decoherence time is somewhat compromised.
Nitrogen-vacancy defects can be formed by irradiated CVD synthetic diamond material which contains single substitutional nitrogen defects to form vacancy defects and annealing the material to migrate the vacancies to pair up with single substitutional nitrogen defects in order to achieve the nitrogen-vacancy defects. Alternatively, under certain growth conditions it has been found that a significant number of nitrogen vacancy defects can be formed directly during growth and these “as-grown” nitrogen-vacancy defects have some advantages over those formed by post-growth irradiation and annealing. For example, as-grown nitrogen-vacancy defects can be preferentially aligned relative to the growth direction of the CVD synthetic diamond material and this preferential alignment can increase the sensitivity of the quantum spin defects in terms of both magnitude and directional sensitivity. Furthermore, due to the fact that no irradiation has been required in order to form the nitrogen-vacancy defects, damage to the diamond lattice may be minimized, and the formation of other defect types that are generated as a consequence of irradiation and/or annealing (e.g. monovacancies and divacancies) which result in a further detriment to the quantum optical properties of the material can be eliminated.
In addition to the presence of nitrogen-vacancy defects, electron donor species are required to convert the neutral defects into negatively charged defects suitable for certain quantum spin defect applications. In this regard, single substitutional nitrogen defects normally functional as electron donating species. As such, a layer of CVD synthetic diamond material containing a high concentration of single substitutional nitrogen and a substantial concentration of nitrogen-vacancy defects may be useful in that the single substitutional nitrogen can donate charge to the nitrogen-vacancy defects to form NV− defects suitable for quantum sensing and processing applications.
One problem with the aforementioned single layer structure is that the single substitutional nitrogen defects can interact with the nitrogen-vacancy defects reducing their decoherence time as previously described. Accordingly, it can be advantageous to provide two separate layers, one containing a large number of single substitutional nitrogen defects so as to function as an electron donating layer and a further layer containing quantum spin defects which can accept negative charge to switch on the quantum spin defects for sensing and processing applications. In this case, the electron donor layer may be provided by using a high nitrogen/high substrate temperature CVD diamond synthesis process.
For optical filter applications, a high concentration of certain nitrogen containing defects having specific optical absorption characteristics can be used to filter light in a controlled manner. Alternatively, for mechanical tool piece applications it has been postulated that high concentrations of certain nitrogen containing defects can improve the wear and/or toughness characteristics of the CVD synthetic diamond material. Further still, as previous described a CVD synthetic diamond material having high concentrations of certain nitrogen containing defects can be used as a starting material for post-growth irradiation and/or annealing treatments to form coloured gemstones. Another potential application of such high nitrogen CVD synthetic diamond material is in lasing applications.
However, the present inventors have identified a problem with such high nitrogen/high substrate temperature CVD diamond synthesis processes. Specifically, the present inventors have found that CVD synthetic diamond material fabricated using such processes is striated under photoluminescent conditions (e.g. using a DiamondView™ imaging technique) due to a non-uniform distribution of nitrogen defects. The striations and non-uniform nitrogen distribution remain even if the CVD synthetic diamond material is subjected to multiple post-growth treatments such high pressure high temperature processing and successive irradiation and annealing treatments.
This is problematic for quantum sensing and processing applications which utilize nitrogen-vacancy quantum spin defects as a non-uniform distribution of nitrogen-vacancy defects within the material results in the material having variable sensitivity. Furthermore, if the material is to be used as a charge donating layer in such applications as previously described a non-uniform distribution of single substitutional nitrogen can result in non-uniform charge donation to another layer comprising quantum spin defects such that the concentration of negatively charged quantum spin defects is non-uniform. Again this results in variable sensitivity.
A non-uniform nitrogen distribution is also problematic for the other applications previously mentioned. For example, in optical filter applications a non-uniform distribution of nitrogen defects will result in non-uniform optical filtering. Similarly, for mechanical tool piece applications a non-uniform distribution of nitrogen defects can result in non-uniform wear and/or toughness characteristics. Furthermore, for gemstone applications a non-uniform distribution of nitrogen defects will result in non-uniform colour thus reducing the quality of the gemstone.
In light of the above, it is an aim of embodiments of the present invention to provide a CVD diamond synthesis process which is capable of forming CVD synthetic diamond material which has both a high and uniform distribution of nitrogen defects. Certain embodiments aim to provide a CVD synthetic diamond material which has both a high and uniform distribution of single substitutional nitrogen defects. Alternatively, or additionally, certain embodiments aim to provide a CVD synthetic diamond material which has both a high and uniform distribution of nitrogen vacancy defects. Alternatively, or additionally, certain embodiments aim to provide a CVD synthetic diamond material which has substantially no visible striations under photoluminescent conditions (e.g. using a DiamondView™ imaging technique).